Use `tssUser` option value for FAPI log directory user in systemd-tmpfiles rules, instead of hardcoding `tss`. If `security.tpm2.abrmd.enable = false` (default), `tssUser` is root. Fixes systemd-tmpfiles-resetup error: /etc/tmpfiles.d/00-nixos.conf:<ln>: Failed to resolve user 'tss': No such process